The invention also relates to an apparatus for diagnosing an injection valve of an internal combustion engine connected to a fuel rail, with a pressure measuring facility, which is configured to measure a fuel pressure in the fuel rail, and with a control facility.
In modern internal combustion engines the fuel to be injected by the injection valves into the combustion chamber of the cylinders of the internal combustion engine is frequently supplied by way of a fuel rail. The fuel rail is connected to a fuel, in particular a high-pressure fuel, supply. Connected in turn to the fuel rail are individual injection valves, which can be actuated to inject certain quantities of fuel by means of suitable control facilities. Such internal combustion engines can be both diesel and gas combustion engines. The injection system can be a so-called common rail injection system for example.
Because of their complex production methods and the different conditions for their deployment, injection valves are subject to major influences in respect of their operating behavior. In particular there is frequently some variance in respect of their operating specifications. Such variances or irregularities cause irregular metering of the fuel mixture and result in the internal combustion engine having higher emissions and not running smoothly, these factors generally being associated with lower efficiency. The variances can be manufacturing tolerances for example, in other words individual deviations of the injection values due to the manufacturing tolerances. Such manufacturing tolerances can be determined by measurement once the valve has been produced and be compensated for by calibration in the engine control unit. Aging phenomena are another type of variance, showing consistent behavior over the service life of the valve, which can be determined for example by long-term measurements so that a modeling of nominal valve behavior can be stored in the control unit.
Two methods are known as equalization functions for injection valves, to compensate for aging phenomena and manufacturing tolerances by adapting the injection time over the entire characteristic flow line of the valve.
One method is the so-called cylinder-selective lambda regulation, which uses one lambda sensor for each exhaust gas bank, said lambda sensor detecting a relative deviation of the cylinders from one another by comparing a cylinder-specific lambda sensor model and the cylinder-specific lambda sensor signal. Assuming that all the cylinders of the internal combustion engine have a regularly distributed air mass flow {dot over (m)}air, it is possible to calculate a mean fuel mass flow {dot over (m)}fuel from the measured lambda value λ and the known stoichiometric ratio c using the following formula:
  λ  =                              m          .                air                                          m            .                    fuel                ·        c              .  
With this known method it is possible to work out the injected fuel mass of each cylinder from the deviation of the cylinder-specific lambda signal from the mean lambda regulator value and adapt the injection correction values on a cylinder-specific basis based on this criterion. However this method cannot be used to diagnose the fuel injectors, as a deviation of the cylinder-specific lambda regulation can originate from both the air and the fuel path and so unique localization of the error site is not guaranteed. This diagnosis method also has limited application in modern turbocharged engines, if the lambda sensor is positioned downstream of the turbocharger.
The second known method uses cylinder-specific uneven running for an adaptation of cylinder-specific injection correction values. The angular acceleration α of the crankshaft, which varies over time, is a measure of the uneven running of the internal combustion engine here and describes the mean induced torque M of each cylinder. The following relationship is used here:M=α·Θ. 
Since the rotational inertia mass θ is considered to be constant, there is a linear relationship between the measurable angular acceleration and the induced torque. With constant ignition parameters and assuming a constant and regularly distributed air mass flow, the mean induced torque is thus obtained as a function of the injected fuel mass by way of each cylinder. The cylinder-specific uneven running is used to modify an individual fuel injection time for the same fuel mass until the deviation from the individual cylinders in respect of uneven running reaches a minimum. This correction is stored in the engine control unit as an adaptation value. However this method cannot be used to diagnose fuel injectors, as a deviation of cylinder-specific uneven running can originate from both the air and the fuel path and so unique localization of the error site is not guaranteed.
With both known methods adaptation values are determined for injection into individual cylinders. Both methods are thus able to correct constant aging phenomena. However they do not offer the possibility of diagnosing a rapidly occurring defect of an injection valve, as unique localization of the error site is not guaranteed.
An apparatus and method for controlling a fuel injector are also known from U.S. Pat. No. 6,964,261 B2. Here a quantity of fuel is injected during a so-called zero fuel condition. A pressure drop in a fuel rail corresponding to the injected quantity of fuel is detected and a change in the engine speed is determined according to the fuel injection. The fuel injection is adjusted as a function of the pressure drop in the rail and the corresponding change in the engine speed. Aging phenomena of the injector can be detected using the known method. However rapidly occurring changes in the injection valve due to a defect are again not taken into account with the method.